Environ. Sci. Technol. 1992, 26,496-502
Interaction of Metals and Protons with Algae. 3. Marine Algae, with Emphasis on Lead and Aluminum Ray H. Crlst, Karl Oberholser, and Jane McGarrlty Messiah College, Grantham, Pennsylvania 17027
DeLanson R. Crlst” Department
of Chemistry,
Georgetown University, Washington, DC 20057
Jill K. Johnson and J. Michael Brlttsan Marine Systems Laboratory, Smithsonian Institution, Washington, DC 20560
The proton uptake rate for intact cells of marine algae is 20 times faster than for freshwater (FW) species, partly due to a higher Na content, but also possibly due to differences in cell wall porosity. Metal displacement by protons on acidification (pH change from 7 to 1)shows two inflection ranges, one a t ca. pH 5 due to the presence of carboxylate anions and the other at pH ca. 1.5 for sulfate groups. Cladophora, Rhizoclonium, and Polysiphonia each have the two inflection ranges, while Enteromorpha and freshwater Vaucheria (FW) each have only one, for sulfate and carboxylate, respectively. Sulfate analysis of Cladophora indicates a sulfate polymer content of ca. 8.2%, which is comparable to that estimated from metal release. Adsorption of Pb and A1 on Rhizoclonium under acidic conditions (metals present as ions) releases H, Ca, and Mg ions from the algae, while adsorption of P b and Cd at pH 10 and A1 at pH 5, where these metal ions are present in very low concentration in equilibrium with solid hydroxide, also releases OH from the hydroxides.
Introduction Algae have an increasingly significant impact on environmental issues ranging from lake putrefaction, sequestering of toxic or precious metals, alternative food sources, and most recently, rescue from the “greenhouse effect”. In many of these issues, metal ions play an important role, one which should be more fully understood. Some of the many problems in understanding the effect of heavy metals arise because of their extensive hydrolysis at the pH of natural waters, and thus these metals are present as hydroxy complexes or insoluble hydroxides and carbonates. In a toxicity-related study, for example, Sylva (1)showed that Cu may exist as seven forms, depending on pH and COz concentration. To demonstrate the availability of adsorbed metals (as on sediments), Kuwabara (2) successfully grew Selenastrium capricornium with the essential Zn adsorbed on Ti02. We found that TiOz itself was adsorbed by Vaucheria (3). Some applications of algae in removal of metals from waters are as follows: concentration or removal of heavy metals ( 4 , 5 ) ;recovery of Au and Ag (6) and uranium (7); control of heavy metals in lead industries (8);Cu transport using acid-tolerant algal species (9). In our work thus far, the emphasis has been on understanding the basic chemistry of the metal adsorption process (10). All work was with intact cells. We have shown that alkali, alkaline-earth, and transition-metal adsorption can be represented by the Langmuir adsorption isotherm, from which a measure of bonding strength and maximum adsorption can be obtained. The process is essentially one of ion exchange with protons and metals of the cell walls, primarily Ca and Mg, the metals available in the limestone springwater medium for the freshwater 496
Environ. Sci. Technol., Vol. 26, No. 3, 1992
(FW) algae studied (11). The equilibrium constants found for the various metals (10) are thus essentially the ionexchange constants for Ca and Mg bonded to carbohydrate anions. Our present work is with marine algae, which have a greatly different growing medium: Na, Ca, and Mg are approximately 0.4,O.Ol and 0.04 M, respectively, compared to 0.0,5 X lo4, and 1.0 X M for the local springwater. The polysaccharide content of marine algae has been investigated extensively. Most of the green species contain both uronate (carboxylate) and half-ester sulfate anions. Marine species are classified into three groups by Percival (12): xylanogalactans, (Cladophora, Rhizoclonium), sulfated glucuronoxylans (Enteromorpha),and glucuronoxylorhamnans (genera noted used in the present work). Alginic acid, an important component of some species, can form gels whose degree of stiffness depends on the divalent metal present (13),and the ion-exchange properties of alginic acid polymers depend on the uronic acid composition (14). In these studies, polysaccharides are hydrolyzed in acid or base to give monomers, which were identified by electrophoresis and chromatography. Specific hydrolysis fractions of Cladophora (15) and of Enteromorpha were found to be high in uronic acid (16). “Water-soluble” polysaccharides were extracted with 90% ethanol. We now report an extension of the basic study of proton-metal interactions with algae to marine species, where Na content is high and the cell wall polymers appear to be considerably different. This is shown in data for proton displacements of metals vs pH, rates of proton uptake, and Langmuir adsorption constants. Also, with our present understanding we now present results of the environmentally important Pb, Cd, and A1 present as ions in the free state (low pH) and at very low concentrations in the presence of the solid hydroxides at pH’s up to 10.
Experimental Section Materials and Methods. The marine algae were obtained from the exhibit Marine Microcosms a t the Smithsonian’s National Museum of Natural History representing the Maine coast (1800 gal, 4-15 “C) and the Carribean coral reef (2500 gal, 25.0-28.5 “C). These exhibition tanks were filled with seawater from Rehobeth Beach, DE. Some molar ion concentrations are as follows: Ca, 0.0013-0.0057; Mg, 0.0400-0.0483; Na, 0.392-0.409; SO4, 0.020-0.022; C1, 0.409-0.471. Each system has a simulated open ocean wave splashing across the tank’s surface every few seconds. The water is circulated through “algal turf scrubber” boxes (17), where a wave dumps across plastic mesh screens irradiated with a multivapor metal halide light (400 W m-2 of screen area). This generates algal growth on the screens, which removes nutrients
0013-936X/92/0926-0496$03.00/0
0 1992 American Chemical Society
and C02 from the microcosms and provides highly oxygenated water back to the systems. Algae were harvested from both the tanks and the scrubbers, stored at 0 “C, and used within -2 weeks. The algae studied were Cladophora serica, Cladophora rupestries, Enteromorpha linza, Enteromorpha intestinales, Rhizoclonium, and Polysiphonia sp. from the Maine coast and also, in preliminary work not reported here, Acanthophora spicifera from the Caribbean coral reef. The Cladophora and Enteromorpha species were used as available. Before use, the marine species were washed three times with deionized water. For algae containing specific metals, e.g., Na, K, and Ca, the sample was suspended in -0.5 M concentration of the metals for 1 h, washed three times, and used directly. Filtrations were accomplished by allowing a suspension to settle, agglomerating the fine particles for a few minutes, and pouring onto a 40-60-mesh stainless steel wire screen. The filtrate was analyzed for metals by atomic absorption on a Perkin-Elmer Model 2380 instrument. Experiments were run in triplicate with data reported on the basis of dry weights. Changes occur when marine species are put into pure water. A metal-anion system reacting with water, e.g., HX Na+ OH-, could account for the NaX H,O --, observed pH’s of 8.5-9.0. Further, osmosis could disrupt the cell walls, liberating cytoplasm. To check the possible effects of osmosis, metal adsorption experiments were run within 5 min and compared with runs over the course of 1h, and no appreciable difference was observed. Also, runs were made with metal blanks taken at each step of the process. Proton Uptake Rates. To determine the rate of proton uptake, an algal sample, 0.10 g (dry weight), was washed three times in water, clipped for better stirring, and suspended in 50 mL of deionized water. The suspension was brought quickly to the desired pH (6, 5, or 4) by adding 0.1 M HC1. In less than 10 s the pH changed due to the bulk solution and a rapid surface reaction was complete (11). The rate of a slow proton uptake (11) was then measured by noting the time and additional amounts of 0.1 M HC1 needed to maintain that desired pH. The slow uptake required 10-20 min, compared to 1h for freshwater species (11). First-order rate constants were calculated using eq 1. For C, one can use the milliliters added for In (C,/C,) = k t (1) the slow process, as this is proportional to moles of HC1 added at time t. The rate constant k. equals the slope of the straight line plot of -In C, vs t. Proton Displacement of Metals. A 0.1-4.2-g sample was suspended in 100 mL of water. The pH at the start varied with the algae, e.g., 8.5-9.5 for Rhizoclonium and 7.0-8.0 for Enteromorpha. The suspension was brought to pH 7.0 by adding HC1, and a 1-mL sample of the liquid was taken for metal analysis. The process was repeated for pH’s 6, 5,4, 3,2.5, 2, 1.5,1.0, and 0.5 with concentrated HC1 being used for the low pHs. After filtration, the dry weight of the sample was obtained. For sulfate analysis of the algae, a 0.5-g sample was ashed and held at 500 OC for 15-30 min, the ash taken up by concentrated HC1, and the sulfate determined as BaSO,. Adsorption of Metals on Rhizoclonium. For Sr, a 0.3-g suspension of Rhizoclonium in 200 mL was stabilized at pH 7.0. A 10-mL sample was removed and filtered, 1 mL of the filtrate being taken for blank analysis. To the remainder was added 0.05 mL of 1.0 M Sr, the pH was checked, and the resulting 9 mL was added back to the 200-mL suspension. A decrease in pH followed by backtitration to pH 7 with 0.02 M KOH gave a measure of
+
+
+
proton displacement. The filtered sample was dried and ashed and the ash analyzed for Sr. The process was repeated for 0.1,0.2,0.4, and 0.7 mL of 1.0 M Sr. The liquid samples were analyzed for displaced Na, Ca, and Mg. The same procedure was used for Cladophora and Enteromorpha. For Pb, one set of experiments was done at pH 5, where the metal is present as Pb2+. Since precipitation of Pb(OH), starts at pH 5.0 and is complete at pH 6.0, pH was carefully controlled as follows. After the whole 200-mL suspension was filtered, a small sample, 0.03-0.04 g, was removed for analysis of Pb adsorbed and 1 mL of the filtrate taken for metal analysis. To the filtrate was added 0.05 mL of 1.0 M Pb(NO,),, and the pH was adjusted to 5.0 if necessary. The sample was now returned to the filtrate. Any decrease in pH was back-titrated with 0.10 M KOH, to obtain the proton displacement. The process was repeated with 0.1, 0.2, 0.4, and 0.7 mL of 1.0 M Pb(NO,),. Experiments were also made with “seawater”with 0.50 M Na, 0.03 M Ca, and 0.05 M Mg, both as chlorides and nitrates, following the Sr procedure. Adsorption of Pb and Cd was measured at pH 10, where the metal ions are present in low concentration in equilibrium with the solid hydroxides. About 0.5 mL of 1.0 M Pb(N03)2or Cd(N03)2in 20 mL of deionized water was taken to pH 10. The precipitate was washed twice with centrifugation and then suspended in 20 mL at pH 10. To this was added 0.2 g of a paper-dried sample stabilized at pH 10, and the released OH was back-titrated with 0.10 M HC1. The filtrate was centrifuged and analyzed for metals, including the ambient Pb2+or Cd2+. For Al, precipitation of A1(OH)3starts at pH 4.0 and is complete at pH 5. The procedure for A1 at pH 4 (A13+) was the same as for Pb at pH 5. The Al solution was taken to pH 5 and an algal sample stabilized at pH 5 added. The pH increased and the released OH was back-titrated with 0.10 M HC1. The solution was analyzed for any released metals. A sample calculation for displacement of protons and magnesium from Rhizoclonium is given in the following. An amount of Pb is added to a suspension of 0.050 g of Rhizoclonium in 200 mL of H20. (a) The pH decreases and 1.0 mL of 0.01 N NaOH is added to take it back to pH 5. Since 1mL of 0.01 N NaOH contains 1.0 X equiv, this is the amount of protons displaced for 0.05 g of alga. For 1.0 g this corresponds to 20 x 10-~ equiv or 200 pequiv g-l. (b) On analysis the solution is found to have Mg at 3.75 X 10-5N concentration. Since 1 mL at this concentration contains 3.75 X lo-* equiv, the 200 mL of solution would have 7.5 X lo4 mol. For a 1.0-g sample this would be 150 X or 150 pequiv 8-l.
Results Proton Displacement of Metals. Metals are released when an algal suspension initially at pH 7 is acidified. The data are shown in Figure 1 for Cladophora and Rhizoclonium and in Figure 2 for Enteromorpha. Values of the sum of metals displaced ( E M ) for Cladophora, Rhizoclonium, and Polysiphonia are presented together with those for Vaucheria in Figure 3. Inspection of the curves for the marine algae in Figure 3 indicates two inflections in the pH range 7-5 and 2.5-1. Vaucheria appears to have the first, while Enteromorpha has the second. Since marine species generally have sulfate ester groups, these data are taken to indicate protonation or a carboxylate anion system at pH 7-5 followed by protonation of sulfate groups at pH 2.5-1. Our preliminary work with other freshwater species shows a similarity to Vaucheria results. Environ. Sci. Technol., Vol. 26,
No. 3, 1992 497
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